A bridge for deer isn't any easier to build than a bridge for pedestrians, even if you don't care about deer being on a bridge that collapses there's a highway underneath. Cars would be crushed and traffic disrupted. So we're just talking about a bridge. It's built to the same standard as any other bridge. (Also it's more like a bridge for mountain lions but whatever.)
If the Empire State Building had been built across a highway, and had to schedule construction around avoiding disruptions to traffic, than it would've taken three years too.
Typical pedestrian bridges aren't 150 feet wide, though. Nor do most have grass growing on them (at least, not intentionally). This is, in fact, a fairly unusual infrastructure project.
Pedestrians also happen to be much denser than cars though, since cars are mostly air, while humans are mostly water, which can lead to some interesting situations (and Wikipedia articles)
My dad worked for a while as an accident investigator for the Air Force. One time there was a crash where a pilot deliberately flew into a cliff at top speed.
The only evidence they ever found of his body was a finger joint. My dad said the body just turns into spray at those speeds.
We went faster when human life had less value. It has more value now, both in labor time (and therefore cost) and harm potential/reduction, so it takes more time and money than it used to. Similar story about disposable workers when the Hoover Dam was built.
We should expect this math to continue as prime working age cohort shrinks over the next century due to structural demographics.
For deer? It's much more for the endangered wild life like mountain lions. I loved in so cal and we had multiple passes like this and it was incredibly helpful in maintaining the mountain lion population
This bridge for deer took 3 times longer to build than the Empire State Building.